1. Field of the Invention
This invention relates to the measurement of a voltage standing-wave ratio and, more particularly to a self-calibrating meter for measuring the voltage standing-wave ratio.
2. Description of the Prior Art
In conventional voltage standing-wave ratio (hereinafter referred to as VSWR) meters, the meter has to calibrated each time the VSWR is measured because the calibration is a function of the transmitted power and frequency. This requires switching the unit to the forward voltage position, turning on the transmitter and adjusting the potentiometer control knob until the meter reads full scale. Then, the switch is moved to the reflected voltage position, the transmitter is turned on again, and the VSWR is read on the meter.
One drawback of the conventional VSWR meter is that it has to be calibrated each time the VSWR is measured, which is time consuming, troublesome and more susceptible to human error in the final reading of the VSWR. In addition, these meters have a potentiometer which has to be turned up when low power is transmitted. This adjustment reduces the resistance applied to the output of the detector diodes because the output voltage of the diodes decreases when the resistance applied thereto decreases. This limits the sensitivity of the conventional meters when transmitting low power. Furthermore, these meters have a lower RC time constant on the output of the detector diodes which limits their sensitivity when making measurements at low frequencies.
In addition to the foregoing, there exists the possible danger, when using the conventional VSWR meters, of burning out the output stage of the transmitter. Since the transmitter must be turned on to calibrate these VSWR meters, if the VSWR is too high, the power reflected back to the transmitter could burn out the output stage of the transmitter. The operator of the meter has no way of knowing if the VSWR is too high because the VSWR cannot be measured with the VSWR meter until after the preliminary calibration procedure is completed.
There are known devices for measuring the VSWR in a transmission line directly with no necessity for calibration or switching. A typical example of such a system is set forth in U.S. Pat. No. 4,041,395 to J. C. Hill. In this patent, a device which incorporates a means for automatically measuring the standing-wave ratio is disclosed. A bidirectional coupler detects voltages which are proportional to the forward and reflected powers. Two operational amplifiers are utilized to calculate the sum and difference of these values. A meter coupled to the output of these amplifier circuits measures a voltage which is proportional to the logarithm of the sum over the difference of the forward and reflected values which is the standing wave ratio. No switching or calibration of the meter is required in the foregoing measuring operation.
Another example of a known technique for measuring the VSWR in a transmission line is disclosed in U.S. Pat. No. 3,842,358 to M. A. Frazier. The VSWR is calculated by processing the outputs of a directional coupler through a differential amplifier. The output of this amplifier, which is proportional to the VSWR, is utilized to control automatic circuitry in the system. A calibration step is not disclosed in this patent.
Although these metering devices do not require calibration or switching while measuring the VSWR, neither accomplishes this result with circuitry such as that of the subject invention described hereinafter in more detail.